Exhaust gases emanating from such devices as internal combustion engines and industrial processes generally contain potentially hazardous compounds such as hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOX) and particulates. Such compounds need to be converted to harmless, or at least less hazardous, compounds in order to reduce the amount of hazardous compounds released to the environment. Commonly the exhaust gases undergo some form of catalytic treatment and/or filtering process.
In most conversion-type treatments of interest in the present context, temperature is an important aspect.
Many important conversion reactions require a rather high temperature. The use of catalysts, for example metals or metal oxides from the platinum group, makes it possible to convert the hazardous compounds with a satisfactory reaction rate at a much lower temperature than if such catalysts were not used. However, a high reaction rate can only be achieved if the temperature is sufficient; that is, above the so called light-off temperature at which the catalyzed reaction rate becomes significant. The light-off temperature is usually in the range 200-400° C. If the light-off temperature has not yet been reached, or if the temperature falls below light-off so that conversion stops, almost no hazardous compounds will be converted. These are well-known problems associated with such things as the cold starting of an internal combustion engine (with a similarly cold catalyzer) and with “cold” exhaust gases, such as those emanating from a diesel engine.
The temperature is further important in regeneration of purification devices, for instance, the removal of trapped particles by combustion or the removal of impurities such as sulphur oxides (SOx) from a catalytic device. Such processes can be cyclic and involve a temperature increase to around 600° C. for a certain time period. As the purification devices normally degrade if they are exposed to overly high temperatures, there is an upper temperature limit that should not be exceeded. Thus, it is not only the temperature that is an important feature, but also the control of the temperature during both conversion (to achieve a good conversion) and during regeneration (to achieve a suitable cleaning of the converter).
A conventional physical structure of a catalytic converter, as for instance disclosed in U.S. Pat. No. 3,885,977, is a ceramic honeycomb monolith with parallel, open channels. The catalytic material is deposited onto the walls of the honeycomb channels. As the gas flows from one end to the other, the catalytic conversion takes place. This type of structure generally works well provided that the temperature of the device is above the light-off temperature. However, at cold-start situations, the hazardous compounds flow through the channels without conversion.
In order to reduce the amounts of hazardous compounds that are released during cold start it is a well known technique to use adsorption traps, i.e. to deposit a material, besides the catalysts, that adsorbs and retains cold hydrocarbons and/or nitrogene oxides until the catalyst reaches the light-off temperature. As an example, this is disclosed in WO95/18292. A problem with this technique when applied to the conventional physical structure described above is that the desorption temperature for most compounds generally is lower than the temperature required for conversion. A great deal of the hazardous compounds will thus still flow through the channels without conversion.
Another approach to solve the problem with cold converters is to introduce electric heating, as disclosed in, for instance WO92/14912. It is, however, difficult to make the heating fast enough and the costs for components and energy are high. This kind of electric heating may also be a safety risk (electricity, fire).
Another important feature is the pressure drop over the purification device as energy is needed to overcome the gas flow resistance of the device. For instance, an increased pressure drop over a purification device for a vehicle engine could result in an increased consumption of fuel.
One technique that has been proposed more recently is the combination of a catalytic purification device and a heat exchanger permitting heat exchange between the incoming gas and the outgoing gas. This technique makes it possible to utilize the heat in the exhaust gas in a more efficient way which is an advantage under most, if not all, operating conditions. EP 1016777 discloses a construction that consists of a corrugated metal strip that is folded onto itself into a bundle that forms gas flow passages between the foldings. However, the shape of the corrugation of the metal strip forms a number of small passages within each larger passage between the foldings, and as the incoming gas flow enters the larger passages from their side, most of the gas will flow in the small passages that are located closest to the side from which the gas was fed. In other words, the gas enters the bundle from the side, and due to difficulties to flow across the larger passages, the gas flow is not distributed within the width of the larger passages. This leads to an overall gas flow distribution that is not uniform. Although this construction is of principal interest, the uneven flow distribution over the catalyst may lead to an insufficient conversion, a less efficient heat exchange and to a high pressure drop over the construction. Furthermore, metal constructions are generally prone to degrade in the rough environment of an exhaust gas flow.